1. Field of the Invention
The present invention relates to a solid-state image sensor, a method of manufacturing the same, and an image pickup apparatus.
2. Description of the Related Art
As a solid-state image sensor, a CMOS (complementary metal oxide semiconductor) solid-state image sensor is widely known. An example of a structure of the CMOS solid-state image sensor is shown in FIG. 9. In FIG. 9, a photodiode 102 and a floating diffusion 103 is formed in a semiconductor substrate 101. The photodiode 102 has a P-N junction structure including an N-type impurity region 104 and a P-type impurity region 105. The floating diffusion 103 is formed of an N-type impurity region 106.
Further, on the semiconductor substrate 101, a transfer transistor 107 is formed. The transfer transistor 107 is structured so that the photodiode 102 and the floating diffusion 103 are set as a source/drain region, respectively. The transfer transistor 107 includes a gate insulating film 108, a gate electrode 109, and spacers 110 and 111. The gate insulating film 108 is formed between the photodiode 102 and the floating diffusion 103. The gate electrode 109 is formed on the gate insulating film 108. The spacer 110 is formed on a sidewall of the gate electrode 109 on a side of the photodiode 102, and the spacer 111 is formed on the other sidewall of the gate electrode 109 on a side of the floating diffusion 103.
The spacers 110 and 111 that cover the sidewalls of the gate electrode 109 are symmetrically formed. Therefore, a length L3 of the spacer 110 and a length L4 of the spacer 111 are set to be the same length (size). In addition, the P-type impurity region 105 is formed in the self-aligned manner with respect to the spacer 110, and the N-type impurity region 106 is formed in the self-aligned manner with respect to the spacer 111.
The length L3 of the spacer 110 and the length L4 of the spacer 111 affect characteristics of a solid-state image sensor. For example, if the length L4 of the spacer 111 on the floating diffusion 103 side is short, a dark current is increased due to a so-called GIDL (gate induced drain leakage). To prevent this, it is desirable to set the length L4 of the spacer 111 to be long to some extent.
On the other hand, if the length L3 of the spacer 110 on the photodiode 102 side is long, pinning of the N-type impurity region 104 of the photodiode 102 is weaken, resulting in the increase in the dark current. Further, if the length L3 of the spacer 110 is short, the dark current is suppressed, but an afterimage is deteriorated. Accordingly, an optimal size of the length L3 of the spacer 110 needs to be set to strike a balance between the characteristics of the dark current and the afterimage.
Japanese Patent Application Laid-open No. 2002-110957 discloses a CMOS image sensor in which a spacer formed on a side of a photodiode is longer than a spacer formed on a side of a floating diffusion.